Biomass-based bioenergy is crucial to meet the goal of making cellulosic biofuels cost-competitive with gasoline. Lignocellulosic materials represent an abundant feedstock for cellulosic-biofuel production. A core challenge in coverting cellulosic material to biofuels such as ethanol and butanol is the recalcitrance of biomass to breakdown. Because of the complex structure of lignocellulosic biomass, pretreatment is necessary to make it accessible for enzymatic attack. Severe biomass pretreatments are required to release the sugars, which along with by-products of fermentation can create inhibitors in the production of ethanol or butanol, for example. During the pretreatment processes, a range of inhibitory chemicals are formed that include sugar degradation products such as furfural and hydroxymethyl furfural (HMF); weak acids such as acetic, formic, and levulinic acids; lignin degradation products such as the substituted phenolics vanillin and lignin monomers. In addition, the metabolic byproducts such as ethanol, lactate, and acetate also impact the fermentation by slowing and potentially stopping the fermentation prematurely. The increased lag phase and slower growth increases the ethanol cost due to both ethanol production rate and total ethanol yield decreases (Takahashi et al. 1999; Kadar et al. 2007).
Efficient conversion of lignocellulosic hydrolysates to biofuel requires high-yield production and resistance to industrially relevant stresses and inhibitors. To overcome the issue of inhibition caused by pretreatment processes, there are two approaches, one is to remove the inhibitor after pretreatment from the biomass physically or chemically, which requires extra equipment and time leading to increased costs. A second approach utilizes inhibitor tolerant microorganisms for efficient fermentation of lignocellulosic material to ethanol and their utility is considered an industrial requirement (Almeida et al. 2007).
Z. mobilis are gram-negative facultative anaerobic bacteria with a number of desirable industrial characteristics, such as high-specific productivity and ethanol yield, unique anaerobic use of the Entner-Doudoroff pathway that results in low cell mass formation, high ethanol tolerance (12%), pH 3.5-7.5 range for ethanol production and has a generally regarded as safe (GRAS) status (Swings and De Ley 1977; Rogers et al. 1984; Gunasekaran and Raj 1999; Dien et al. 2003; Panesar et al. 2006; Rogers et al. 2007). One drawback to using wild-type Z. mobilis is its narrow substrate utilization range. However, recombinant Z. mobilis strains have been developed to ferment pentose sugars such as xylose and arabinose (Zhang et al. 1995; Deanda et al. 1996; Mohagheghi et al. 2002). A newly formed partnership between the DuPont and Broin companies utilizes recombinant Z. mobilis strains for bioethanol fermentation from the lignocellulosic residues such as corn stover (Reisch 2006). On the other hand, low tolerance to acetic acid and decreased ethanol tolerance have been reported in recombinant strains (Ranatunga et al. 1997; Lawford and Rousseau 1998; Lawford et al. 2001; Dien et al. 2003).
Acetic acid is an inhibitor produced by the de-acetylation of hemicelluloses during biomass pretreatment. At pH 5.0, 36% of acetic acid is in the uncharged and undissociated form (HAc) and is able to permeate the Z. mobilis plasma membrane (Lawford and Rousseau 1993). The inhibition mechanism has been ascribed to the ability of the undissociated (protonated) form to cross the cell membrane leading to uncoupling and anion accumulation causing cytoplasmic acidification. Its importance comes from the significant concentrations of acetate that are produced relative to fermentable sugars (McMillan 1994) and the ratio of acetate to fermentable sugars is particularly high in material from hardwoods (Lawford and Rousseau 1993). Acetate may reach inhibitory levels when pretreated biomass hydrolysates are concentrated to generate high final ethanol concentrations or where process water is recycled. Acetate removal processes have been described but they are energy or chemical-intensive and their impact on processing costs have yet to be determined (McMillan 1994).
An acetate tolerant Z. mobilis mutant (AcR) has been generated by a random mutagenesis and selection strategy (Joachimstahl and Rogers 1998). The AcR mutant was capable of efficient ethanol production in the presence of 20 g/L sodium acetate while the parent ZM4 was inhibited significantly above 12 g/L sodium acetate under the same conditions. A number of studies have characterized the performance of recombinant Z. mobilis strains able to utilize both C-5 and C-6 sugars, including under acetate stress conditions (Lawford et al. 1999; Joachimsthal and Rogers 2000; Lawford and Rousseau 2001). Acetic acid was shown to be strongly inhibitory to wild-type derived strain ZM4 (pZB5) on xylose medium and nuclear magnetic resonance studies indicated intracellular deenergization and acidification appeared to be the major inhibition mechanisms (Kim et al. 2000). A recombinant strain able to utilize both xylose (a C-5 sugar) and glucose (a C-6 sugar) with increased acetate resistance was generated by transforming plasmid pZB5 into the AcR background (Jeon et al. 2002). Mohagheghi et al. (2004) reported a recombinant Zymomonas mobilis 8b tolerated up to 16 g/L acetic acid and achieved 82%-87% (w/w) ethanol yields from pure glucose/xylose solutions.
Acetic acid bacteria are used for the industrial production of vinegar and are intrinsically resistant to acetic acid. Although the resistance mechanism is not completely understood, progress toward this goal has been made in recent years. Spontaneous acetic acid bacteria mutants for Acetobacter aceti (Okumura et al. 1985) and several Acetobacter pasteurianus strains (Takemura et al. 1991; Chinnawirotpisan et al. 2003) showed growth defects in the presence of acetic acid, which was associated with loss of alcohol dehydrogenase activity. Fukaya et al (1990) identified the aarA, aarB, and aarC gene cluster as being important for conferring acetic acid resistance using a genetic approach (Fukaya et al. 1990). aarA encodes citrate synthase and aarC encodes a protein that is involved in acetate assimilation (Fukaya et al. 1993) and the three aar genes have been suggested to support increased flux through a complete but unusual citric acid cycle to lower cytoplasmic acetate levels (Mullins et al. 2008). The presence of a proton motive force-dependent efflux system for acetic acid has been demonstrated as being important in A. aceti acetic acid resistance, although the genetic determinant(s) remain to be identified (Matsushita et al. 2005). In E. coli, over-expression of the ATP-dependent helicase RecG has been reported to improve resistance to weak organic acids including acetate (Steiner and Sauer 2003). Baumler et al. (2006) describe the enhancement of acid tolerance in Z. mobilis by the expression of a proton-buffering peptide in acidified TSB (HCl (pH 3.0) or acetic acid (pH 3.5)), glycine-HCl buffer (pH 3.0) and sodium acetate-acetic acid buffer (pH 3.5) (Baumler et al. 2006). Baumler et al. (2006) also note that the presence of the antibiotic also significantly increased acid tolerance by an unknown mechanism.